In recent years, the demand for using, in process control systems, programmable electronically controlling devices which are not only limited to the control of primary plant functions (general functions) but also encompass control with respect to safety functions regarding human life and the environment has intensified. In the control of safety functions, safety is demanded, so for that reason, for one safety indicator with respect to data communication between devices, error detection matching is used and error correction is performed.
However, there is demanded an improvement in the missed error detection rate, the probability of not detecting an error even with error detection, so in order to implement safety, technology has been devised in which two or more frames including data and error detection matching are received and a matching comparison of error detection codes is performed. This kind of technology is described e.g. in JP-A-2005-49967.
Moreover, in the draft of Functional Safety Standard IEC 61508, there are mentioned, as primary factors obstructing safety, resending of the communication path, loss, insertion, erroneous order, delay, and masquerading (forgery), so countermeasures against these are demanded. For these, there is demanded a reduction in the missed error detection rate, the probability of not detecting an error even with error detection, so in order to implement safety, there has been devised the technology of carrying out a matching comparison of safety data having an important influence on the system. This kind of technology is described e.g. in JP-A-1986-134135.
Also, with the advancement of technology in the electronics and information fields, the application range for programmable electronic devices is becoming wider, driven by the increased complication/compositeness of functions demanded of single devices, and at the same time, the reliability demanded of programmable electronic devices is increasing.
In the midst of the progress in increasing scale and the integration of plants and the execution of highly automated plant operation, there are problems with the spread of international safety standards and a lack of experienced persons, and further improvements in safety, beyond the safety measures built up in the past, are in the process of becoming a necessary condition, so, as clearly defined in the functional safety standard IEC 61508-1 to -7, “Functional Safety of Electrical/Electronic/Programmable Electric Safety Related Systems”, Parts 1 to 7 (IEC 61508/61511, JIS C 0508), it is regarded as important to prevent and alleviate, in their respective layers, the occurrence of accidents and the extension of damage.
As far as control devices are concerned, in case an anomaly is detected, the system is required, in order to satisfy the aforementioned functional safety standard, to operate with certainty, and even in the unlikely event of a breakdown, it is demanded to stop the processes on the safe side, so the functional safety system needs to have a special design with great importance attached to “safety” different from that of the control system.
Also, in large-scale control systems, distributed control systems with process input/output devices having sensors installed in the vicinity of the process and controllers installed in a control room slightly separated from the process are becoming the mainstream, so it is becoming important, as far as functional safety is concerned, to find out how to prevent faulty operation of the process input/output device due to error in data communication between the controller and the process input output devices.
As one of the most common error detection methods in data transmission, there is CRC (Cyclic Redundancy Check), as described in JP-A-1999-74869.